The Lora module is not provided with an antenna. So a 17.3cm or 16.4cm (0.95 velocity factor applied) can be soldered to the ANT pi (especially useful when prototyping) or an SMA connected for connection to a larger gain antenna such as a Yagi Uda.

Calculation of wavelength

The RFM96W works on the 70cm bandwidth, that's 433Mhz. So to get the wavelength we need to do a bit of Maths. The equation we need is:

v=f\lambda v is velocity - which in this case is the speed of light or 299,792,458 m/s f is frequency - which in this case is 433MHz or 433,000,000Hz

so re-arranging for wavelength we get:

299,792,458/433,000,000 = 0.69236m This gives us a wavelength of 69.24cm. We approximate this to the 70cm band, which is an amateur radio band in the UHF part of the spectrum.

Transmitter

Theory

So, what kind of antenna do we need?

The most basic is a 1/4 wave vertical antenna. Dividing the wavelength of 70cm by 4 gives us a basic antenna of length 173mm. Simply attaching a 173mm length of wire will give us a basis antenna. This is probably all that is needed for the CanSat.

The 1/4 wave antenna is going to be are starting point for the transmitter. If we wanted to test the efficiency of the antenna we would need to get into the theory and practice of managing the Voltage Standing Wave Ratio and velocity factors. This is a bit of overkill for this exercise but is useful background knowledge to understand resonance and antenna systems.

Before we leave this it is worth noting that this type of antenna is omnidirectional. It radiates equally in all directions perpendicular to the plane. The image below from Antenna Theory website gives us the idealised radiation pattern. Note that the orientation of the antenna, which is half of a half wave dipole i.e. a quarter wave is vertical. If the antenna is on its side then the pattern is rotated 90 degrees also.

Receiver

Theory

The receiver is going to be on the ground. The quarter wave antenna might just work. But we also might want to have a bit of directivity and some way of improving the signal, or gain. For this a common type of antenna is called a Yagi, or Yagi-Uda, after the inventor(s). This antenna type has the ability to have forward gain and to help reject signals from other directions. They are commonly seen on roof tops as TV antennas. The UHF TV bands in the UK are quite close to the 70cm band in fact, so sizes are quite similar. The antenna is made up of a reflector at the rear, a driven element and one or more directors. The greater the number of directors the greater the gain (with diminishing returns) and also the increase in directivity. So for high gain antennas, you need to point the quite accurately to get the best out of them.

A similar plot from the Antenna Theory for the Yagi design is as below.

To calculate the antenna size we need the same information as before, namely the frequency at which we are going to operate i.e. 433MHz. So, to calculate the lengths of the reflector, driven and director elements we can use a Simple Online Caluclator to do the sums for us. Note that there are many tweaks to the original design which alter boom lengths, driven element configuration to include matching systems and so these are approximations. As his is a receive only antenna we can largely ignore some aspects but we will cover these anyway.

*** Matching and VSWR or SWR

To get the most out of and antenna and to reduce any likely damage to a transmitter the antenna system needs to be resonant. If the system is not resonant then there are implications. The general term for this is Impedance matching. It gets complicated very quickly but the simple rule of thumb is that transceivers are generally designed around 50 Ohm impedance (over the air TV systems are an obvious exemption to this as they are 75 Ohms). Our connectors, cables & antenna like to be at 50 Ohms. Anything outside of that and we start to introduce losses which we note as VWSR or SWR (depending on how you like to see it).

Our connectors (in the case of the RFM96W are SMA type) are 50 Ohms. One less thing to worry about. Our coaxial cable is 50 Ohms. But a feature of coaxial cable is that it is unbalanced. You get a bonus that it is easy to manage but a minus with the unbalanced nature. the length of cable can affect the impedance though. Our antenna is 50 Ohms and resonant by design and balanced - Uh Oh! The cable is unbalanced but the antenna is balanced If we were to transmit directly into the unbalance system we would typically see some of the energy return back down the coaxial cable and result in RF not being where we need it. In poor systems that results in energy that can affect surrounding environment as noise, in really poor systems it can do strange things like turn off the TV or affect computer systems when powers are higher than what we are dealing with.

To get the most out of a system there needs to be a matching network. There are many ways of doing this, however for a receive only antenna we can probably ignore this. If performance is not what you were expecting then consider adjusting the antenna design to include a matching network.

Testing

Primary mission was tested using BMP280 and Lora transmitted on a breadboard. Antenna used was a 16.4cm wire soldered to the LORA module. Lora receiver was connected to a second breadboard. Antenna used was once again a 16.4cm wire soldered to the LORA module. Data received successfully indoors (tried 2 adjacent rooms and room across a corridor) and outdoors in direct line of sight for about 100m in school. Need to test in open grounds.

Yagi Uda

Design

Build

Theory

Testing

A simple Yagi was built using 3 elements made of cut lengths of an old tape measure. The distance between them was calculated using this calculator. We used a piece of scrap timber for the boom. Once constructed, the Yagi seemed so small but this was expected as per the theory. We would attach it to a longer stick/insulator when using it.

Research

Good guide to antenna selection

Quarter wave vertical antenna

Example tape measure Yagi with PVC boom

How to strip a coaxial cable

How to connect SMA

Dipole research and resonant antenna

More info about RTL

[Good link about antenna resonant frequency, bandwidth and gain] (https://www.electronics-notes.com/articles/antennas-propagation/antenna-theory/resonance-bandwidth.php)

Lessons learnt

• Ensure that the wire antennas are both vertical.
• Scratch the paint off the tape measure elements of the Yagi before soldering wire.
• Watch 'How to connect an SMA connector to an RG174 cable'
• Order SMA connectors with pins already soldered from RM0MT ebay
• Buy a crimp tool if possible
• Can use wago connectors to connect feedline to Yagi
• SDR - like a radio receiver See noise level and strength of signals above it Now try with Yagi with SMA connector

Tips: • Contrast to min, both sides blue • Change f to 433 000000 • Click on play • Must remember to change source to usb rtl